The invention takes as its starting point an electrically controlled braking system which is intended for a wheeled vehicle.
A braking system of this kind is known from U.S. Pat. No. 4,802,562, FIG. 9. As is customary, the input means are there formed by a brake pedal, which is pressed down by a driver of a vehicle in accordance with the retardation of the vehicle he requires. The brake pedal is assigned two sensors, which can, for example, be designed as rotary potentiometers. Each sensor records the extent of actuation of the brake pedal and outputs an electrical output signal corresponding to this extent. In the case of the known braking system, two electronic control units which operate independently of each other are furthermore provided, one control unit being supplied with the output signal of one sensor and the other control unit being supplied with the output signal of the other sensor at the brake pedal as an input signal. Each wheel of the vehicle is assigned a brake cylinder and a braking pressure modulator valve which is fluid-connected to the brake cylinder and has an electric actuating element, namely a piezoelectric actuator. The two electric actuating elements of two braking pressure modulator valves can only be activated by one control unit and the two actuating elements of two other braking pressure modulator valves can only be activated by the other control unit. If one control unit fails, therefore, only two braking pressure modulator valves continue to operate.
DE 31 44 961 A1 has disclosed a braking system for a wheeled vehicle in which actuating a brake pedal adjusts a pneumatic or hydraulic pilot control valve and an angle transmitter. A braking pressure modulator valve can be adjusted, on the one hand, pneumatically, or hydraulically by means of a pressure fed in by the pilot control valve or by an electromagnet which is activated in accordance with the position of the angle transmitter. In this braking system, if one control circuit fails the braking pressure modulator valve can still be actuated with the aid of the other control circuit.